Casein Kinase 2 (CK2) is a serine/threonine enzyme involved in the increment of the cell proliferation, being the nucleus it main intracellular location during the malignant transformation process. (Tawfic S., Yu S., Wang H., et al. (2001) Protein kinase CK2 signal in neoplasia. Histol. Histopathol. 16:573-582). Moreover some key viral proteins for the pathogeny of Human Immunodeficiency Virus (HIV) and of Hepatitis C Virus (HCV) have been reported as CK2 substrates (Meggio F., Marin O., et al. (2001) Mol Cell Biochem 227:145-151; Franck N., Le Seyec J., et al. (2005) Hepatitis C virus NS2 protein is phosphorylated by the protein kinase CK2 and targeted for degradation to the proteasome. J Virol. 79:2700-2008).
Findings of other groups worldwide, had also confirmed the existence of elevated levels of CK2 in different solid tumours of epithelial origin, in orders ranging from 3 to 7 times higher, respect to the normal tissue. (Tawfic S., Yu S., et al. (2001) Protein kinase CK2 signal in neoplasia. Histol Histopatol. 16:573-582; Faust R. A., Gapany M., et al (1996) Elevated protein kinase CK2 activity in chromatin of head and neck tumors: association with malignant transformation. Cancer Letters 101:31-35), besides the phosphorylation activity of the CK2 enzyme being a very important event in the malignant transformation of the cells it constitutes a strong marker on the tumour progression, (Seldin D. C., Leder P. (1995) Casein Kinase IIα transgene-induced murine lymphoma: relation to theileroiosis in cattle. Science 267:894-897), the over expression of CK2 on the other hand leads to the tumorigenesis of mammary cells by the up regulation in the signalling cascade Wnt/beta-catenin (Landesman-Bollag E., Romien-Mourez R., et al (2001) Protein Kinase CK2 in mammary gland tumorigenesis. Oncogene 20:3247-3257). Recent findings also suggested that CK2 plays an essential role in some other processes like chromatin remodelling (Barz T., Ackenmann K., et al. (2003) Genome-wide expression screens indicate a global role for protein kinase CK2 in chromatin remodelling. J Cell Sci. 116:1563-1577) and the regulation of cell survival (Unger G. M., Davis A. T., Slaton J. W., Ahmed K. (2004) Protein kinase CK2 as regulator of cell survival: implications for cancer therapy. Curr Cancer Drug Targets, 4:77-84). Of paramount importance for the understanding of the cancer development process, had been the findings proving that CK2 mediated phosphorylation is a very strong signal for cell survival, therefore considering this enzyme as an antiapoptotic mediator for the cell physiology (Ahmed K., Gerber D. A., Cochet C. (2002) Joining the cell survival squad: an emerging role for protein kinase CK2. Trends Cell Biol, 12:226-229; Torres J., Rodriguez J., et al (2003) Phosphorylation-regulated cleavage of the tumour suppressor PTEN by caspase-3: implications for the control of protein stability and PTEN-protein interactions. J Biol Chem, 278:30652-60).
On the basis of the foregoing findings, the CK2 mediated phosphorylation has been confirmed as a biochemical event, suitable to be used as a potential target for the therapeutic intervention on cancer, rendering all potential inhibitors of such event as prospective candidates for the treatment of such condition. Up to date several research groups worldwide have been developing different strategies to inhibit CK2 mediated phosphorylation with two experimental approaches: a) The direct inhibition of the CK2 enzyme, or b) The blocking of the phosphorylation site near to the acidic domain described as common to all CK2 substrates.
For both approaches, the authors have been able to demonstrate the concept by which the inhibition of the CK2 mediated phosphorylation event, yield to the induction of apoptosis on tumour cells, which implies an experimental validation of CK2 as a very promissory target in the finding of drugs for cancer treatment.
Example of the latter is a direct inhibitor of the enzyme like the 4,5,6,7-tetrabromotriazole (TBB) tested as a potent apoptosis and caspase dependent degradation inducer in Jurkat cells on the micro molar concentration range. (Ruzzene M., Penzo D., Pinna L. (2002) Protein kinase CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) induces apoptosis and caspase-dependent degradation of haematopoietic lineage cell-specific protein 1 (HS1) in Jurkat cells. Biochem J., 364:41-47). Also, by inhibiting the expression of the CK2 enzyme by using anti-sense oligonucleotides, an in vitro apoptotic effect and antitumoral action in an experimental cancer model in mice. (Guo C., Yu S., et al. (2001) A potential role of nuclear matrix-associated protein kinase CK2 in protection against drug-induced apoptosis in cancer cells. J Biol Chem, 276:5992-5999; Slaton J. W., et al. (2004) Induction of apoptosis by antisense CK2 in human prostate cancer xenograft model. Mol Cancer Res. 2:712-721).
Other compounds like antraquinone derivatives, flavonoids, and halogenated azobenzylimidazoles have been described as CK2 ATP binding site inhibitors (Sarno S., et al. (2002) Toward the rational design of protein kinase casein kinase-2 inhibitors. Pharmacol Therapeutics 93:159-168), and 5-oxo-5,6-dihidroindolo(1,2-a)quinazolin-7-yl acetic acid (IQA) have been reported as a selective CK2 inhibitor using high throughput screening, (Vangrevelinghe E., et al. (2003) Biochemical and three-dimensional-structural study of the specific inhibition of protein kinase CK2 by [5-oxo-5,6-dihydroindolo-(1,2-a)quinazolin-7-yl]acetic acid (IQA). J. Med. Chem. 46:2556-2662).
The aforementioned compounds have shown their CK2 activity inhibiting effect in the micro molar range for the Inhibitory Concentration 50 (IC50), but no evidences have been reported of any antitumoral action in experimental models of cancer.
The other approach to inhibit the CK2 activity have been to interfere with the phosphorylation site on the substrate, in the patent application WO 03/054002 and the work of Perea S. E et al. (2004) Antitumor effect of a novel proapoptotic peptide impairing the phosphorylation by the protein kinase CK2. Cancer Res. 64:7127-7129, the authors are limited to propose the use of a cyclic peptide family to block the CK2 mediated phosphorylation on the substrate site, showing tumour cell cytotoxicity and antitumoral effect in cancer pre-clinical models. However the described peptides have the limitation of not being able per se to penetrate into the cells, hence requiring a membrane permeation peptide being fused to them.
In general terms, when compared to small molecules, the use of peptides have the drawbacks of a decreased in vivo stability in the circulation, degradation, being very difficult to formulate in oral administration and they are not easily transported inside the cells. (Ludger Wess, Isogenica: Improving peptides, Biocentury Oct. 25, 2004). Other problems of peptides widely described in the literature are the faster clearance, their immunogenic potential, and their cost per therapeutic dose is known to be generally superior to non-peptidic drugs.